In the past few years, with the dramatic increase in applications of power supply-sensitive electronic equipment in the field of process control and automation, the problem of power supply quality has attracted more and more attention. This is because this problem will result in process interruption, and bring about considerable economic losses to industrial customers. The voltage fluctuation is considered to be the most destructive one among the problems of power supply quality which can affect the power supply quality of a power distribution system. Series custom power compensation is regarded as a desirable and efficient method for improving the power supply quality due to its fast response and high reliability. For example, a device utilizing such a method is a dynamic voltage restorer (DVR), which can reduce the effects of voltage fluctuations on the power supply side by injecting energy into a downstream sensitive load.
A hardware structural diagram of a typical DVR used in a single-phase system is shown in FIG. 1.
As shown in FIG. 1, the DVR is mainly composed of the following parts:
a voltage source converter (VSC): the VSC controlled by pulse width modulation (PWM) or other modes is the most important constituent part in the DVR, and it is used to synthesize an injection voltage by utilizing a power-electronic device (such as IGBT, MOEFET, etc.) in a pulse width modulation inverter. In the present application, a typical structure of the VSC is listed as follows: three single-phase fullbridge VSCs, each of which comprises four pairs of an IGBT (G) and a diode (D). This patent application can also be utilized in VSCs with other structural types.
An energy storage: a direct current (DC) end of the VSC is to be connected to an energy source or an energy storage device with an appropriate capacity. The energy storage device will supply necessary energy to the VSC, and it can generate an injection voltage. Such an energy storage can be shared by three phases together, and it is also possible to have one energy storage for each phase. This energy storage can be an energy storage device such as a battery, a large capacity capacitor, etc.
An isolation transformer: the purpose for including the isolation transformer is to boost the injection voltage generated by the VSC and to couple the injection voltage into a feeding circuit. The coil ratio of a boosting transformer has been chosen carefully so as to enable the VSC to compensate any flash voltage sag with a minimal DC connection voltage. In some DVR designs, the isolation transformer can be omitted.
A harmonic filter: the output of the VSC controlled by PWM will contain a large quantity of high-order harmonics due to the high on-off frequency of the VSC. Therefore, it is necessary to use a harmonic filter system so as to supply a cleaned injection voltage on the primary side of the isolation transformer. Normally, the filter system is composed of an L-C part, and the value of the filter element will rely on the frequency of the voltage of high-order harmonics generated by the VSC.
In the figure, VS and VL represent respectively the voltages at the power supply side and the load side of the DVR. These values are easily measured. The DVR installed between a protected load and its power supply bus can use the injection voltage Vinj from the DVR to cancel the voltage fluctuations appearing in VS, so as to ensure a high-quality load voltage.
As to the DVRs used in three-phase three-wire or three-phase four-wire systems, if only their hardware structures are considered, without considering the possibility that they share a common energy storage, they can actually be regarded as a combination of three single-phase DVRs. The hardware structure of a three-phase DVR is shown in FIG. 2. In FIG. 2, there is a by-pass switch connected between the power supply side and the load side, for choosing whether or not to connect the DVR.
FIG. 2 shows clearly a three-phase DVR for monitoring the phase voltages ({right arrow over (V)}Sa, {right arrow over (V)}Sb, {right arrow over (V)}Sc) at the power supply side. Once a voltage fluctuation appears in the system, the DVR injects the voltage components ({right arrow over (V)}inja, {right arrow over (V)}injb, {right arrow over (V)}injc) into each phase, so as to restore the phase voltages ({right arrow over (V)}La, {right arrow over (V)}Lb, {right arrow over (V)}Lc) at the load side to the nominal level, and this can be described as:{right arrow over (V)}La={right arrow over (V)}Sa+{right arrow over (V)}inja {right arrow over (V)}Lb={right arrow over (V)}Sb+{right arrow over (V)}injb {right arrow over (V)}Lc={right arrow over (V)}Sc+{right arrow over (V)}injc  (1)
Due to the use of three single-phase DVRs for reducing voltage fluctuations in conventional three-phase power supply systems, the costs thereof are relatively high.